This invention relates generally to oscillators, and more particularly to an adjustable frequency oscillator.
Adjustable frequency oscillators are used in diverse applications. These applications include receivers, transmitters, data transmission lines, cellular phone systems, and optical and analog communication systems. Very often these applications include phase locked loops (PLLs). PLLs are often used to lock a local clock signal of a receiver to a clock signal of a transmitter. Adjustable frequency oscillators are used in PLLs to generate an adjustable clock signal.
Several architectures have been proposed for adjustable frequency oscillator designs. These architectures may largely be categorized as varactor-based oscillators, RC-tuned oscillators, interpolation oscillators, oscillators based on other variable elements, or resistor sensing oscillators. Each of the above categories of oscillators have several inherent shortcomings.
Perhaps the most widely used type of an oscillator is a varactor-based oscillator. A varactor based oscillator is an LC tank circuit with variable capacitance. The resonant frequency .omega..sub.n of an LC tank circuit is 1/LC. Therefore, by varying the capacitance C of an LC tank circuit, the resonant frequency may be varied. The capacitance is generally varied using a varactor. A varactor is an adjustable capacitor, and is often formed using a reverse-biased diode. A reverse-biased diode has a pn junction with an inherent capacitance. The inherent capacitance is related to the width of the depletion area of the pn junction, and this depletion area varies with the level of reverse-biasing. Accordingly, the inherent capacitance of the pn junction may be modified by changing the reverse bias of the diode.
Unfortunately, the capacitance of the reverse-biased pn junction is inherently non-linear, and exhibits somewhat linear behavior only over small voltage ranges. Therefore, a range of reverse-biasing voltages of the pn junction at which the capacitance varies in a linear manner must be determined for varactor applications. This limits the capacitance range of a varactor, and thus a range of oscillator frequency. Because of this limitation, in order to use the varactor-based oscillator over a wide range, some form of ranging with switching between a plurality of varactors is required to allow increased capacitance variation. This, however, occurs at the expense of increased circuit complexity.
In addition, a varactor has a capacitance proportional to the voltage across it. As the varactor charges up and down during oscillation, the voltage across the varactor changes, proportionately changing the capacitance of the varactor. Therefore, the resonant frequency of the oscillator changes as it oscillates. Furthermore, varactors are not a common element in many designs and therefore tend to be poorly modeled as compared with transistors.
One other disadvantage of a varactor-based oscillator is a non-linear frequency vs. voltage characteristic. Even if the capacitance C of a varactor is linearly proportional to the voltage across it, the oscillation frequency .omega..sub.n will be proportional to 1/V since .omega..sub.n =1/LC and C=C.sub.0 +V*K.sub.CV. This non-linearity can be a problem in PLL applications since most PLL models and equations are only valid for linear oscillators. One way to reduce this non-linearity problem is to apply a correction function to the voltage before the oscillator (for example, squaring and inverting V), but this further complicates the circuit and introduces other variables into the circuit. Another method of overcoming the non-linearity problem is to assume that over the range of V, 1/V is approximately linearly proportional to V. This approximation, however, is only good for narrow ranges of V.
Another commonly used oscillator circuit is based on an RC-tuned circuit with an RC time constant. A voltage rise time and a fall time of a capacitor in an RC-circuit are related to the RC time constant, and therefore a rate of change of voltage across the capacitor, and thus a passband of an oscillating signal output of the oscillator circuit, is also related to the RC time constant. In a typical RC-tuned circuit, a variable resistor is used to vary the RC time constant. There are several types of RC-tuned circuits found in applications such as ring oscillators. The main disadvantage of RC-tuned circuits is that Q (Quality factor of resonance) is undefined for the RC-tuned circuits. Because of their lack of Q, RC-tuned circuits have higher phase noise, and are usually highly susceptible to power supply noise.
Sometimes two LC tank circuits with different oscillation frequencies are coupled together to form a single oscillator, thereby forming an interpolator. In interpolators the oscillation frequency of the oscillating signal output is varied by giving a different weight factor to the output of each LC tank circuit. Oscillators with an interpolator have a reasonable Q factor, but this Q factor changes quite a bit over frequency, and is also non-linear. If the output frequency of the interpolator is same as the resonant frequency of one of the two interpolated LC tanks, the Q is high. However, if the output frequency differs significantly from the resonant frequencies of both LC tanks, the Q decreases.
Another technique used for oscillator applications is the use of a variable impedance circuit (VIC) to change the value of a capacitor. This technique has several problems. First, the VIC is a tuned circuit itself, requiring design in which the tunable range matches the range of the oscillator. It also lowers the Q of the capacitor since it introduces real components to the current. This is highly undesirable since it then lowers the Q of the circuit.
A resistor sensing mechanism in resistor sensing oscillators places a resistor in series with either an inductor or a capacitor of an LC tank circuit. This mechanism takes advantage of the fact that ac currents tapped from two ends of the inductor or capacitor are out-of-phase with each other. This mechanism reduces Q (quality factor of resonance) of the oscillator enough to make it undesirable for integrated circuit applications.